Hydrostatic power transmissions are commonly used for many purposes such as, for example, vehicular drives. These transmissions utilize a variable displacement hydraulic pump connected by fluid conduits to a hydraulic motor to vary the speed of the motor. Medium duty transmissions may require power assist to stroke the transmission. Such power assists may be in the form of a hydraulic piston such as shown in U.S. Pat. No. 3,650,107. One problem with such power assists is that one or more pistons of the power assist engage a yoke to vary the displacement of the pump. However, the forces on the yoke at these points of displacement are usually too high to permit manual stroking of the pump, if necessary. Another shortcoming of the current power assists being used is the lack of "feel", that is, a machine operator receives no sense of the work effect through the control linkage.
Among the objectives of the present invention are to provide a hydraulic pump with servo displacement control which obviates the aforementioned problems by providing manual stroke, if necessary, and which provides a sense of feel. The control combines servo and actuation functions. In other words, servo functions are obtained as long as hydraulic control pressure is maintained; however, if that hydraulic control pressure is ever lost, manual actuation of the yoke to stroke the pump is obtained through the control. A rotary vane actuator provides yoke turning torque and is combined with a port plate to provide the servo function. The combination of the servo and actuation functions reduces package size, part count, cost, utilizes a minimum number of parts, allows manufacturing at low cost and provides an assembly that can be readily mounted on a pump.
In order to provide the operator with feel, a torsion bar is incorporated into the control and is anchored to one end of the yoke pintle so that a portion of the yoke forces will be transmitted through the torsion bar to the operator.
Another advantage of the integral type servo control is the reduction and simplification of control linkages. The integral servo control eliminates yoke vibrations transmitted to the control linkage that occurs during the damping action of the stroking pistons. Yoke vibrations which are transmitted to the control linkage are a major source of noise. Therefore, the servo control not only eliminates yoke vibrations but also reduces noise.
In accordance with the invention, the servo displacement control is a stand alone sub-assembly designed to be readily bolted onto an existing device. The servo control combines actuation and servo functions and incorporates a torsion bar to provide an operator with feel for the amount of work being performed. The servo control also has a fail safe feature which allows manual stroking of the yoke in the event that the control pressure is lost.